Expert: Tom Whiting - 12/15/2006

Question
Hi there,

I have a question:
What are temperatures in the highest layers of atmosphere, and lets say if there is an animal or human being would it get frozen or? and why.

Thanks
Meti

Answer
Hi Meti
Actually this is a meteorological (weather) question, but no matter as I taught Meteorology too.
The highest level of the atmosphere is called the thermosphere, a very rarified region of our atmosphere, extending from the top of the mesopause up to, and into,
outer space.  The mesopause is at 90 Km altitude (about 50 miles high) and real outer space begins at about 200 Km
(about 120 miles) high.  The air temperature at the mesopause
is around -90 C but the the temperature rises dramatically
as you climb from 50 to 120 miles.  Of course, there is actually
no heat....recall that temperature is defined as the motion or speed of the atoms and molecules, whereas heat is the "quantity of energy" function.   And at the mesopause, you
are above 99.999% of the atmosphere.
{The tip of a cigarette and a campfire are both the same
temperature, but a lit cigarette won't keep you warm at night!} In effect, the thermosphere is just a very
warm vacuum but only because the atoms and molecules
of air are really zipping around fast in a near-vacuum condition.   As far as freezing, either animals or humans,
the air temperature would be the least of your problems.
Your fatal exit would be....lack of air pressure, thus your
blood and all body fluids would immediately change to the
vapor state (boil) if you were suddenly thrown into the thermosphere without a pressure suit on.  
{Recall that as the pressure drops over a fluid, the boiling
point also drops...water boils, ...converts to the vapor state,
at 212 degrees F at sea level, but more like 180 degrees
F in the mountains at 11,000 feet high!}
Indeed, at any altitude beginning at around 65,000 feet (12 miles high) you have to have either a pressurized cabin, or a pressure suit on, to survive, because the boiling point of
water, blood, body fluids, etc... drops to 98.6 degrees F.    Even above 40,000 feet, (8 miles high) you have to be breathing pressurized air (or oxygen) in order to survive at that low air pressure, because the lungs cannot adequately exchange carbon dioxide/oxygen across the lining in the lungs.  (ie..the partial pressure of oxygen becomes too low).

As far as the freezing point, in the summer time the freezing
level is generally right around 10,000 feet (2 miles high)
but it drops to more like 7,000 feet in the winter months.
So yes, if you or an animal were placed at 20,000 feet,
where the temperature is typically minus 40 C (or -40 F)....
a body would freeze.  But as I stated above, at the really
high levels of the atmosphere, like the stratosphere, mesosphere, or thermosphere,.....it's the lack of air pressure
that kills any unprotected animal or human, and not the air temperature. One has to be in a pressurized container, or spacesuit., to survive.
Hope all this helps,
Tom Whiting   (former USAF Pilot)
Erie,  PA


PS....thus, air pressure is really our FIRST survival requirement, even before oxygen, temperature, water,
and food.  With zero air pressure, we are all dead!!